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Transistor Switch — Parallax Forums

Transistor Switch

David KissickDavid Kissick Posts: 15
edited 2005-02-13 01:23 in General Discussion
Hi everyone,
Not exactly Basic Stamp related, but this has been a problem that I have been unable to find a solution to. I'm creating a device to work with a Playstation 2 (A DDR pad for those familiar with it). The idea is to press down on a panel with your foot and create a circuit by connecting the power and ground from one button on the controller. Well, that works great. Now, I would like to have that panel light up when you press down. The first thought was to have a transistor turn on two LEDs. However, I either get only lights or only the button working depending on the arrangement. I attached a drawing of the current circuit.

What I believe is happening is if I don't put the battery in, then the controller work fine since it's going from the base straight to ground and getting the full 3.5V that the controller puts out. However, if I put the batter in, then the voltage out of the transistor is 0, which saturates the transistor but does not work the controller. What do I need to do to make this work? Should I try something else, or just need a different arrangement? I would appreciate any help. Hope the drawing is clear. It's just a basic transistor switch. Thanks!

David

Post Edited (Gecko Studio) : 2/11/2005 11:05:09 PM GMT
circuit.gif
320 x 320 - 3K

Comments

  • MacGeek117MacGeek117 Posts: 747
    edited 2005-02-11 23:04
    David-you might what to draw your circuit in Paint-just use the lines.
    If you draw the circuit with the post, it will get crushed.
    bugg

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    I think I know what I'm doing...
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-02-11 23:10
    Thank you. It should work- what is the voltage of the battery-and do you have a logic probe?
    bugg

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    I think I know what I'm doing...
  • David KissickDavid Kissick Posts: 15
    edited 2005-02-11 23:11
    Yes, I noticed what happened to my drawing, so I've added a gif file. That should help.
  • David KissickDavid Kissick Posts: 15
    edited 2005-02-11 23:14
    The batter is 9 volts (The LEDs have a voltage drop of 3.8V, and I use two in series though I only drew one). I'm not familiar with a logic probe, but I know I don't have one. About all that is there is what you see in the drawing. As stated before, if I don't have the batter in place, then its just a straight connection from the controller power to controller ground and works fine. If I put the battery in, then the transister turns on and powers the lights when the switch is closed, but the controller doesn't respond (ie, the signal doesn't go high). Hope that helps. thanks for the quick response.

    David
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-02-11 23:17
    You're welcome. You might also what to try this:
    connect a 10k Ohm resistor between the controller and ground.
    bugg

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    I think I know what I'm doing...
  • David KissickDavid Kissick Posts: 15
    edited 2005-02-11 23:34
    Bugg,
    Thanks for the suggestion. I tried that, the controller works, but the LEDs only flicker for a brief moment and don't come on. I tried higher values, but at about 100k only the lights work. It seems to be either-or at this point. I'm positive this can be done, but just not sure how. Somehow, I need to be able to send the original control voltage back to the controller, but run the lights off the battery and control both with only one switch. Any other suggestions? thanks again for the speedy help.

    David
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-02-11 23:36
    Just for kicks, try 4.7k Ohm. (yellow-violet-red)
    bugg

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    I think I know what I'm doing...
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-02-11 23:41
    Gecko- try a 4049 inverter and a PNP transistor.(3906 should work)
    bugg

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    I think I know what I'm doing...
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-02-11 23:45
    You might also what to try to make your own DDR pad out of DIP switches.
    bugg

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    I think I know what I'm doing...
  • David KissickDavid Kissick Posts: 15
    edited 2005-02-11 23:45
    Thanks again. I'll look into that.
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-02-11 23:48
    You're welcome.
    bugg

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    I think I know what I'm doing...
  • David KissickDavid Kissick Posts: 15
    edited 2005-02-11 23:53
    Oh, I missed the other replies. So here's some answers:

    1. A 4.7k resistor makes the controller work, but not the LEDs.
    2. The switch on the pad works fine. I'm just trying to add light to it. this was one suggestion given (to use a transistor) but no one seemed to know how to make it work. The lights are just a visual, and if I can't find a way to add them, then all's not lost, just not as flashy wink.gif

    If anyone comes up with other ideas, let me know. Bugg, again thanks for the help. I owe you one.

    David
  • KenMKenM Posts: 657
    edited 2005-02-12 12:39
    If the power from the controller is capable of supplying 100 micro amps (and that is likely the case) the posted circuit will work as long as the switch remains closed.

    FYI, the configuration is called an emitter follower.

    Not drawn, but you seem to be aware from your original drawing, the ground for the 9v battery and the ground from the power source need to be tied together.

    Ken
    emitter follower.jpg
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  • David KissickDavid Kissick Posts: 15
    edited 2005-02-12 13:44
    Hey, thanks for the reply. I never got around to trying it becuse I found this circuit late at night. I works great, though it seems a little odd to me with the battery reversed, but I'm not going to complain about a working circuit. Anyway, I thought I would post this to pass on the info others. Thanks again for all the help.

    David
    switch.gif
    320 x 320 - 2K
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-02-12 19:03
    You haven't followed KenM's schematic completely.·Your are operating your transistor in the reverse active mode of operation. The collector of bipolar transistors are longer than the emitter to reduce hot injection effects and other issues. Since you only using the transistor for switching there isn't any serious problem in doing this (in small signal amplification this is more of an issue) but you can end up reducing the lifetime of your transistor (though not by a hugh amount).
  • KenMKenM Posts: 657
    edited 2005-02-13 00:03
    One very important point I missed with the emitter follower circuit I posted.

    The circuit I posted assumed that the voltage to the switch would be 9 volts.

    If the voltage to the switch is less than 8.2 volts, the circuit "should not" work. Even at 8.2 volts the current through the LEDs will be minimal.

    The reason is that when using an emitter follower, the emitter voltage will be one diode drop (~0.6) volts less than the base voltage.

    Each LED using 3.8 volts X 2 = 7.6 volts, plus 0.6 = 8.2 volts, but very little current through the LEDs because of the 330 ohm resistor.
  • David KissickDavid Kissick Posts: 15
    edited 2005-02-13 01:23
    Thanks guys!

    I wondered if the arrangement would do anything to the transistor. However, being used in a switch format I figured that it would limit any wear on it. A little bit of shortened life span isn't a big deal (it'll be in an easy to replace area). thanks for the info on that Paul. Ken, thanks for the background on the circuit. That arrangement may work well for other projects, so I'll keep it in mind. So glad to have a great group of people to help out this lost soul!

    David
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